1. (Field of the Invention)
The present invention generally relates to a sensor-equipped bearing assembly having a rotational speed sensor incorporated therein and, more particularly, to the sensor-equipped bearing assembly used in and in the vicinity of a magnetic field of a relatively high intensity such as generated in a general purpose electric motor.
2. (Description of the Prior Art)
The sensor-equipped bearing assembly is well known in the art. By way of example, the conventional sensor equipped bearing assembly 51 is shown in FIG. 8, which includes an inner race 52, an outer race 53 accommodating the inner race 52 therein with an annular bearing space defined between it and the inner race 52, and a row of rolling elements 54 retained by a retainer or cage 55 and interposed between the inner and outer races 52 and 53 within the annular bearing space. A ring-shaped encoder 56 is fixed to one of the inner and outer races 52 and 53, for example, the inner race 52, that is rotatable, and a magnetic sensor 57 which may be a Hall element or the like is secured to the other of the inner and outer races 52 and 53, for example, the outer race 53, that is stationary, in face-to-face relation with the ring-shaped encoder 56. The magnetic encoder 56 is in the form of a rubber magnet having N and S poles magnetized therein so as to alternate with each other in a direction circumferentially thereof. This magnetic sensor 57 is housed within a resin casing 58 and is then resin-molded. Securement of this magnetic sensor 57 to the outer race 53 is made by rigidly securing the resin casing 58, with the magnetic sensor 57 therein, to the outer race 53 through a metallic casing 59.
According to the conventional sensor-equipped bearing assembly of the structure discussed above, as the inner race 52 rotates relative to the outer race 53, the magnetic sensor 57 detects change in polarity of the magnetic encoder 56 then rotating together with the inner race 52 and then to output a detected output signal in the form of a train of pulses as shown in FIG. 9. The pulse signal outputted from the magnetic sensor 57 provides an indication of not only the number of revolutions of the inner race 52, but also the direction of rotation of the inner race 52 relative to the outer race 53. The sensor-equipped bearing assembly of the type referred to above is compact in size and robust and requires no complicated assembling adjustment and is accordingly widely used in various electric motors for supporting a drive shaft.
The conventional sensor-equipped bearing assembly discussed above is disclosed in, for example, the Japanese Laid-open Patent Publication No. 2002-174258.
However, it has often been found that when the sensor-equipped bearing assembly of the structure shown in FIG. 8 is placed in a magnetic circuit of a magnetic coil or magnet capable of generating a magnetic field of a relatively high intensity, the sensor-equipped bearing assembly 51 tends to provide an erroneous output under the influence of leakage fluxes resulting from an external magnetic field.
By way of example, with reference to FIG. 10, the situation will be discussed in which the sensor-equipped bearing assembly is incorporated in an electric motor for rotatably supporting a drive shaft 62. In this illustrated example, a rotor 61 mounted on the drive shaft 62 for rotation together therewith is rotatably supported by a housing 63 by means of a front bearing assembly 64 and a rear bearing assembly 65 represented by the sensor-equipped bearing assembly. A stator 60 is fixed to the housing 63 so as to encircle the rotor 61. In this structure, when a high electric current is supplied to the stator 60, flow of magnetic fluxes cannot be neglected and as shown by the arrow in FIG. 10 a magnetic loop is created that extends from the stator 60 back to the stator 60 through the rotor 61, then through the drive shaft 62, through the inner race 52, through the outer race 53 and finally through the housing 63. As a matter of course, when the electric current flows in a direction reverse to that described above, the magnetic loop correspondingly reverses in direction.
At this time, since except for the rolling element 54 and the retainer 55, the annular bearing space between the inner and outer races 52 and 53 is dominantly occupied by non-magnetic elements and provides a high magnetic resistance, some of the magnetic fluxes tend to leak and the resultant leakage fluxes adversely affect the magnetic sensor 57.
For the magnetic sensor 57, a combination of a hall element, a Hall IC (integrated circuit) constructed of an integrated circuit for converting an output signal from the Hall element into a digital signal and a MR element (magnetic resistance element) is generally employed. FIG. 11 illustrates an exemplary inner structure of the Hall IC, which includes a Hall element 71 for detecting a magnetic field, an amplifier circuit 72, a Schmitt trigger circuit 73 and an output transistor 74. While the Hall IC is available in two types, a switch type that is switched on and off depending on the strength of the magnetic field and a alternating magnetic field type that is switched on and off when S and N poles of the magnet are alternately applied, the alternating magnetic field type is generally employed in the rotation sensor. Hereinafter, undesirable influences brought about by the leakage fluxes on the sensor-equipped bearing assembly will be discussed in reference to the example in which the Hall IC shown in FIG;. 11 is employed.
When no leakage flux exist, as shown in FIG. 12, an alternating magnetic field is applied to the Hall IC of the magnetic sensor 57 as the magnetic encoder 56 shown in FIG. 8 rotates. An analog signal thus outputted from the Hall IC is converted into a pulse signal having ON and OFF states alternating each time the analog output exceeds a threshold value, which pulse signal has a duty ratio (Tp/Tn) of about 50%.
When leakage fluxes act externally, as shown in FIG. 13A, the alternating magnetic field applied to the Hall IC of the magnetic sensor 57 offsets upwardly or downwardly depending on the direction in which the leakage fluxes act. Because of this, the output from the Hall IC of the magnetic sensor 57 has its duty ratio varying as shown in FIGS. 13B and 13C. Also, the amount of offset increase with increase of the leakage fluxes and may result in drop-out of one or some of the output pulses and/or failure of the Hall IC to provide the output.